As taught in U.S. Pat. No. 4,011,129 issued Mar. 8, 1977 to Tomlinson, it is known to burn a portion of the black liquor from the kraft pulping in a fluidized bed oxidation zone and to direct the pellets formed during this fluidized bed oxidation onto the surface of the char bed in a conventional kraft recovery boiler and there to reduce the sodium sulfate in the pellets to sodium sulfide. This process has proved quite satisfactory and is currently being practiced at Domtar's Fine Paper Mill in Cornwall, Ontario. The reduction efficiency of the furnace has been found to have been, to some extent, adversely affected by the additional load applied thereto through the application of pellets on to the bed.
It is also well known to pyrolize a residual pulping liquor and subsequently burn the gas generated while simultaneously burning the remainder of the organics in the liquor, thereby to generate sodium carbonate pellets and to subsequently contact the sodium carbonate pellets with SO.sub.2 derived from the burning of the remainder of the gas from the pyrolizer to form sodium sulfate (see Canadian Pat. No. 960,811, issued Jan. 14, 1975 to Shick).
It has also been proposed in U.S. Pat. No. 4,135,968 issued Jan. 23, 1979, to Dehass, to dry a portion of the black liquor from a kraft mill, pyrolyze same and add the dry pyrolyzed solids to the remainder of the black liquor for injection into the recovery furnace whereby the inorganics in the pyrolyzed solids as well as in the remainder of the liquor are reduced in the recovery furnace. The gas from the pyrolyzer will generally be treated for sulfur removal and then burned in a separate unit.
In another arrangement dry residual liquor solids are pyrolized to produce a combustion or fuel gas from about 70% of the organics present in the dried liquor and the remaining organics and the inorganics are treated in a molten salt bath gasifier to produce further fuel gas and a smelt. The gasifier may be operated under reducing conditions so that the smelt when dissolved provides a green liquor. The fuel gases produced in the pyrolizer and the gasifier may be burned to generate steam in a boiler.